New Delhi - Millions of primary schoolteachers and other government employees started knocking at doors across India Wednesday in what the Census Department claims is the world's biggest headcount.

The massive exercise is carried out every 10 years, with this year's involving 2.7 million census takers.

According to the 2001 census India had a population of 1.02 billion. It is estimated to have grown to at least 1.2 billion.

The provisional results are due to be announced by the end of March by Registrar General and Census Commissioner C Chandramouli.

The census takers have to each knock on 124 to 300 doors over the next 20 days to cover the country's 240 million households. They will get paid 6,000 rupees (132 dollars) for the task.

Teachers are favoured as enumerators - the technical term for the head-counters - because they have the easiest access to households and get better responses when they ask personal questions, regarding for example age and sex.

For the first time India's census gives its transgender residents the option of 'other,' alongside the boxes for male and female.

The first phase of the 2011 census, counting the households, was done between April and September 2010. The current phase is the population count.

The third is a controversial caste census, which is opposed by several Indian political parties. While supporters say the caste data will help affirmative action programmes, critics says it would lead to divisive politics.

The caste census is being done for the first time since 1931 and is scheduled for between June and September 2011.

The figures involved in the population count are mindboggling. With a budget of 22 billion rupees, it is to cover all citizens in the country's 7,742 towns and cities, and 600,000 villages.

Chandramouli said the exercise is very cost-effective with the expenditure per person coming to just 18 rupees.

The enumerators will not just count people but collect information on their religion, languages spoken, occupation, education, standard of living, and other details.

The questions in the data sheets include the ownership of mobile phones, computers, access to drinking water and internet and whether a person has a bank account.

The data yielded by the census is compiled, collected, sorted and published - a process which has so far taken five or six years - and is used by administrators, policymakers, corporations, researchers and social scientists.

While the numbers become part of the public domain, the information relating to each individual is kept confidential. 'It is not even accessible to a court of law,' Chandramouli said.

Data on members of the migrant population will be collected at airports, railways stations, ports and bus terminals on the night of February 28.

A second round of counting is scheduled between March 1 and 5 to cross-check the data.

The omissions rate in the Indian census is usually around 1.7 per cent, though it did jump to 2.3 per cent in 2001, which census officials say was because it was digitized for the first time.

The count in some areas may face difficulties, with Maoist rebels controlling areas in West Bengal in the east of the country and in the central state of Chhattisgarh, the Business Standard newspaper reported citing government officials.

Some primitive tribes on the Andaman and Nicobar island chains are considered too hostile to be counted by going door-to-door. But the enumerators have a plan to draw them out into the open, where they can be counted from a safe distance.

In the Andamans' Sentinel Islands, for example, officials said they plan to throw coconuts and brightly coloured cloth into the sea and film the tribe members when they come to collect them at the shore, and later take the census from the recordings.

Efforts are on to reduce the omissions rate and to hasten the data crunching process by using advanced technology. 'We hope to process all the data within two years this time,' Chandramouli said.

The latest census data is also to be used to help the ongoing Unique Identification (UID) programme aimed at creating a biometrics-based system of identification cards for all Indians above the age of 15.